High speed steel manufactured by power metallurgy

ABSTRACT

A high speed steel which has been manufactured power metallurgically and has the following chemical composition in weight-%: 0.6-0.9 C, from traces to max 1.0 Si, from traces to max 1.0 Mn, 3-5 Cr, 0-5 Mo, 0-10 W, where (Mo+W/2) shall be at least 4, 0.7-2 V, max 14 Co, 0.7-1.5 Nb, with the balance being substantially iron, incidental impurities and accessory elements in normal amounts. The steel is suited for tools the use of which require a high toughness, a suitable hardness and strength.

BACKGROUND OF THE INVENTION

The invention relates to a new high speed steel suited for tools the useof which requires a high toughness in combination with a hardness andstrength suitable for the application in question. Typical applicationsare dies for the extrusion of aluminum profiles, qualified machineelements and pressure rolls, i.e. tools for embossing patterns orprofiles in metals, etc. Another field of application is tools forcutting working, e.g. thread cutting taps and end-cutters with chipbreakers, which require a high toughness in combination with a highhardness, particularly a high hot hardness.

One of the most important features of a steel which shall be used, e.g.,for tools for the extrusion of aluminum profiles is that the steel istempering resistant, which means that it shall be able to be exposed toa high temperature during a long period of time without losing thehardness which the steel has obtained after hardnening and tempering. Onthe other hand, this hardness need not be extremely high, suitably beingin the range 50-55 HRC.

A high hardness and strength in combination with a high toughness areprimary features if the steel instead shall be used for qualifiedmachine elements. In this case, the hardness after tempering typicallymay be in the range 55-60 HRC.

Still higher demands upon hardness, 60-67 HRC, even in combination witha high toughness are raised on steels for tools intended for embossingpatterns or profiles in metals, etc., and on steels for tools forcutting working, i.e. thread cutting taps and end cutters with chipbreakers. Thread cutting taps should have a hardness in the range 60-65HRC while end cutters should have a hardness in the range 62-67 HRC.

For applications of the above mentioned kind there are contemporarilyusually used such tools steels as hot work steels, qualifiedconstructionsteels and sometimes high speed steels. An example of a highspeed steel for this type of application is the commercial high speedsteel which is known under its trade name ASP® 23 (currently availablefrom Erasteel Kloster Aktiebolag, a Swedish corporation), which ischaracterized by the following nominal composition in weight-%: 1.29 C,0.4 Si, 0.3 Mn, 4.0 Cr, 5.0 Mo, 6.2 W, 3.1 V, balance iron andunavoidable impurities. Another high speed steel which is used, e.g.,for cutting working, is ASP® 30 (currently available from ErasteelKloster Aktiebolag, a Swedish corporation), which has the nominalcomposition 1.28 C, 4.2 Cr, 5.0 Mn, 6.4 W, 3.1 V, 8.5 Co, balance ironand unavoidable impurities. All percentages relate to weight-%.

The above-mentioned commercially available steels ASP®, 23 and ASP®, 30have a considerable toughness in comparison with other high speed steelsbut do not completely satisfy those demands which are raised onmaterials, e.g., for the above mentioned applications, nor do any othercommercial steels currently exist which fully satisfy all the saiddemands. The purpose of the invention is to provide a new high speedsteel which more satisfactorily fulfills these demands. Moreparticularly, the steel shall have the following features:

it shall have a high toughness in the hardened condition;

a hardness of max 250 HB before hardening;

a good hardenability, including precipitation hardenability to ahardness between 50 and 67 HRC suitable for the application in question,by choice of a hardening temperature between 925 and 1225° C. andsubsequent tempering; and

a high toughness in the hardened and tempered condition by the fact thatthe steel contains a comparatively small total amount of carbides, max5% by volume, that the carbides are small and evenly distributed, thatthe microstructure is fine grained corresponding to austenite grainshaving sizes corresponding to Intercept>20 according to Snyder-Graff),and that it has a low content of retained austenite.

These and other conditions may be satisfied if the steel is given abalanced alloy composition according to the appending claims. In thefollowing, the choice of the various alloy elements will be discussed.Herein, some theories will be mentioned concerning mechanisms which areconsidered to be the basis for the achieved effects. It shall, however,be noted that the claimed patent protection is not bound to anyparticularly theory.

DESCRIPTION OF THE INVENTION

Carbon has several functions in this steel. Above all, carbon shallexist to a certain amount in the matrix in order to afford the matrix asuitable hardness through the formation of martensite by cooling fromthe dissolution temperature and to an amount sufficient for thecombination of carbon with in the first place molybdenum/tungsten andvanadium during tempering after the dissolution treatment for theachievement of precipitation hardening by the formation of M₂ C- andMC-carbides, respectively. Carbide also exists in the steel in the formof niobium carbide which is not dissolved at the hardening process butcan work as grain growth inhibitors in the grain boundaries of themicrostructure of the steel. Therefore, the carbon content in the steelshall be at least 0.6% and preferably at least 0.65%, :suitably at least0.67%. On the other hand the carbon content must not be so high that itwill cause brittleness. The maximal carbon content in the steeltherefore generally is 0.85%, at least for those applications which donot require significant amounts of cobalt in order to afford the steel ahigh hot strength, preferably max 0.8%, suitably max 0.78% C. If thesteel contains a high content of cobalt in order to provide a desiredhigh hot hardness, e.g. if the steel shall be used for tools for cuttingworking, the carbon content may lie on a somewhat higher level, suitablymax 0.9%, as the cobalt may have an influence upon the content ofretained austenite, so that this readily may be converted to martensitewhen tempering. The nominal carbon content is 0.75% when the steel shallbe used for products at the use of which a hardness in the range 58-65HRC, preferably at least 60 HRC, is desired, e.g. for embossing tools.If the steel instead shall be used e.g. for tools for the extrusion ofaluminum profiles, a hardness higher than 50-58 HRC, preferably max 55HRC, is not required. In this case a nominal carbon content of 0.70% maybe more suitable. One may also conceive a nominal carbon content of0.73% for products which shall have a hardness between or overlappingthese extremes, or 55-60 HRC, e.g., for qualified machine elements. Ifthe steel shall be used for tools for cutting work, which tools requirea high hot hardness so that the steel ought to contain cobalt in higheramounts and a hardness in the range 62-67 HRC, the nominal carboncontent suitably is 0.80%.

Silicon may exist in the steel as a residue from the deoxidation of thesteel melt in amounts which are normal from the metallurgicaldeoxidation praxis, i.e., max 1.0%, normally max 0.7%.

Manganese may also exist in the first place as a residue from themelt-metallurgical process-technique, where manganese has importance inorder to make sulphur impurities harmless, in a manner known per se,through the formation of manganese sulphides. The maximal content ofmanganese in the steel is 1.0%, preferably max 0.5%.

Chromium shall exist in the steel in an amount of at least 3%,preferably at least 3.5%, in order to contribute to a sufficienthardness of the matrix of the steel. Too much chromium, however, willcause a risk for retained austenite which may be difficult to transform.The chromium content therefore is limited to max 5%, preferably to max4.5%.

Molybdenum and tungsten shall exist in the steel in order to bring abouta secondary hardening effect during tempering after solution heattreatment because of the formation of M₂ C carbides, which contribute tothe desired wear resistance of the steel. The ranges are adapted to theother alloying elements in order to bring about a proper secondaryhardening effect. The content of molybdenum may be max 5% and thecontent of tungsten max 10%, preferably max 6%, and in combinationMo+W/2 shall be at least 4%. Normally each of molybdenum and tungstenshould exist in an amount of 2-4%, suitably 2.5-3.5%. In principal,molybdenum and tungsten wholly or partly may replace each other, whichmeans that tungsten may be replaced by half the amount of molybdenum andmolybdenum be replaced by double the amount of tungsten. One knows,however, from experience that approximately the same proportions ofmolybdenum and tungsten are preferable on the present total level ofthese alloying elements, since this gives some production technicaladvantages, more particularly advantages relating to the heat treatmenttechnique.

The total amount of M₂ C-carbides which can be produced in the steelstructure at the precipitation hardening treatment is limited. In orderfurther to increase the hardness and wear strength of the steel aftertempering, the steel alloy therefore also shall contain vanadium whichcombines with carbon at the tempering operation to form MC-carbides,wherein the secondary hardening is augmented through precipitationhardening. In order to obtain a sufficient effect, the content ofvanadium should be at least 0.7%, suitably at least 0.8%. The content ofvanadium, however, must not be too high in order that none-dissolvedprimary vanadium carbides may not be retained after the solution heattreatment, which retained primary carbides could impare the toughnessand at the same time bind carbon intended for the precipitationhardening. Therefore the vanadium content is limited to max 2%,preferably to max 1.5%, suitably to max 1.3%.

The matrix of high speed steels known in the art having a compositioncomparable with that of the present invention will be brittle because ofgrain growth at the hardening from a high temperature, since the majorpart of the carbides are dissolved at the solution heat treatment.Conventionally a high toughness therefore is achieved by hardening froma lower temperature so that there will be a sufficient amount ofcarbides in the steel to inhibit the grain growth. This, however, at thesame time implies that one has had to accept a lower hardness. Thisproblem according to the invention is solved by two processes:

firstly, the steel is alloyed with niobium and with a sufficient amountof carbon--as far as carbon is concerned, se supra--in order to providea sufficient amount of niobium carbides, NbC, which are not dissolved toa substantial degree at the above mentioned, high temperature but willremain undissolved to work as grain growth inhibitors.

secondly, measurements are taken in order that the primary niobiumcarbides be small and evenly distributed in the steel, which is acondition for their ability to work as grain growth inhibitors. Thiscondition is satisfied by the powder-metallurgical manufacturing, whichgarantees that the niobium carbides be small and evenly distributed.

An amount of niobium in the steel suitable for the functioning ofniobium as grain growth inhibitors under the above mentioned conditionsis 0.7-1.5%, suitably 0.8-1.3%. Lower amounts of niobium do not providea sufficient grain growth inhibiting effect, while higher amounts maycause embrittlement.

The possible presence of cobalt in the steel is determined by theintended use of the steel. For applications where the steel normally isused at room-temperature or where the steel is not heated toparticularly high temperatures during use, the steel should not containintentionally added cobalt, since cobalt reduces the toughness of thesteel. However, cobalt can be tolerated in amounts up to max 1.0%,preferably max 0.5%. If the steel shall be used for tools for cuttingwork, wherein the hot hardness is of primary importance, it is on the,other hand suitable that the steel contains significant amounts ofcobalt, which in that case should exist in an amount between 2.5 and14%, suitably max 10% cobalt, in order to provide a desired hothardness.

Besides the above mentioned elements, the steel contains nitrogen,unavoidable impurities and other residual products in normal amountsderived from the melt-metallurgical treatment of the steel. Otherelements can intentionally be supplied to the steel in minor amounts,provided they do not detrimentally change the intended ineractionsbetween the alloying elements of the steel and also that they do notimpair the intended features of the steel and its suitability for theintended applications.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further explained in the following descriptionwith reference to performed experiments and to achieved results. Hereinreference will be made to the accompanying drawings, in which

FIG. 1 shows the hardness after tempering versus the hardeningtemperature;

FIG. 2 shows the hardness versus the temperature;

FIG. 3 shows the bending strength versus the hardness; and

FIG. 4 shows the toughness expressed as deflection prior to ruptureversus the hardness.

The composition of the examined steels are given in Table 1. Besides thealloying elements given in the table, the steels only contained iron andimpurities and accessory elements in normal amounts. All the steels No.1-7, except steel No. 2, were manufactured powder-metallurgically in theform of 200kg capsules, which were consolidated to full density throughhot isostatic pressing at 1150° C., 1 h and 1000 bar. Steel No. 2 wasmanufactured conventionally in the form of an ingot. From the capsulesand from the ingot, respectively, there were made rods with thedimension 100 mm φ through conventional hot rolling. Steel Nos. 8 and 9are reference materials corresponding to the commercially steel gradesASP® 23 and ASP®, 30, respectively.

                  TABLE 1                                                         ______________________________________                                        Steel                                                                         No.   C      Si     Mn   Cr   Mo   W    V    Nb   Co                          ______________________________________                                        1     0.51   0.43   0.28 4.2  3.0  3.1  1.41 --   0.03                        2     0.60   0.49   0.31 3.9  3.0  2.9  1.20 --   0.02                        3     0.81   0.53   0.30  4.14                                                                               3.03                                                                               3.07                                                                              1.00 1.09 --                          4     0.75   0.48   0.31  3.99                                                                               2.99                                                                               3.07                                                                              1.01 1.10 --                          5     0.70   0.69   0.30  3.97                                                                               3.05                                                                               3.06                                                                              0.99 1.16                             6     0.83   0.37   0.34 4.1  2.9  3.0  1.1  1.1  0.32                        7     0.80   0.48   0.27 4.0  3.0  3.0  1.0  1.1  7.9                         8     1.29   0.40   0.30 4.0  5.0  6.2  3.1  --   --                          9     1.28   0.50   0.30 4.0  5.0  6.4  3.1  --   8.5                         ______________________________________                                    

Steel Nos. 3-9 were hardened through solution heat treatment attemperatures varying between 1050 and 1250° C. (steel No. 4 between 950and 1250° C.), cooling to room-temperature and tempering at 560° C. Thesolution heat treatment was made during 3 min, while the tempering,which was repeated three times, was made during a holding time of 60min. The achieved hardnesses versus the hardening temperature (thetemperature for the solution heat treatment) are shown in FIG. 1.

In the second series of experiments with the same steels, the temperingtemperatures varied between 500° and 600° C. In this case test specimenswere used which had been hardened from 1180° C. The hardness versus thetempering temperature is shown in FIG. 2.

In the third series of experiments the bending strength versus thehardness of the steels 2-5 and 7-9 were examined. The results are shownby the curves in FIG. 3.

Finally the toughness of the same steels versus the hardness in afour-point bending test was examined. Cylindrical test rods were bent torupture. The deflection at rupture was measured, which is a measurementof the toughness. The results are shown by the diagrams in FIG. 4.

FIG. 1 ;and FIG. 2 show that it is possible to obtain a suitablehardness of the steels of the invention for the conceived applicationsafter tempering if a suitable hardening temperature between 925 and1250° C. is chosen. FIG. 3 and FIG. 4 show that the best strength andthe best toughness are achieved with the niobium containing steels ofthe invention, particularly with steels No. 4, No. 5 and No. 7.

I claim:
 1. High-speed steel manufactured powder-metallurgically andcomprising the following chemical composition in weight-%:

    ______________________________________                                        0.6-0.9      C                                                                from traces to max 1.0                                                                     Si                                                               from traces to max 1.0                                                                     Mn                                                               3-5          Cr                                                               0-5          Mo                                                               0-10         W, where (Mo + W/2) shall be at least 4                          0.7-2        V                                                                max 14       Co                                                               0.7-1.5      Nb                                                               ______________________________________                                    

with the balance being substantially iron incidental impurities andaccessory elements.
 2. High-speed steel according to claim 1, comprisingthe following chemical composition in weight-%:

    ______________________________________                                                0.6-0.85     C                                                                from traces to max 1.0                                                                     Si                                                               from traces to max 1.0                                                                     Mn                                                               3-5          Cr                                                               2-4          Mo                                                               2-4          W                                                                0.7-1.5      V                                                                max 1.0      Co                                                               0.7-1.5      Nb                                                       ______________________________________                                    

with the balance being substantially iron, incidental impurities andaccessory elements.
 3. Steel according to claim 1, comprising thefollowing chemical composition in weight-%: 0.6-0.8% C, max 1.0% Si, max1.0% Mn, 3.5-4.5% Cr, 2.5-3.5% Mo, 2.5-3.5% W, 0.8-1.3% V, max 1.0% Co,0.8-1.3% Nb.
 4. Steel according to claim 3, comprising the followingchemical composition in weight-%: 0.65-0.8% C, max 1.0% Si, max 1.0% Mn,3.7-4.3% Cr, 2.7-3.3% Mo, 2.7-3.3% W, 0.8-1.3% V, 0.8-1.3% Nb.
 5. Steelaccording to any one of claims 2-4, comprising in weight-% 0.67-0.78% C.6. Steel according to any one of claims 1-4, comprising in weight-% max0.5% Si and max 0.5% Mn.
 7. Steel according to claim 1, comprising thefollowing chemical composition in weight-%:

    ______________________________________                                                0.6-0.9      C                                                                from traces to max 1.0                                                                     Si                                                               from traces to max 1.0                                                                     Mn                                                               3-5          Cr                                                               0-5          Mo                                                               0-10         W                                                                0.7-2        V                                                                2.5-14       Co                                                               0.7-1.5      Nb                                                       ______________________________________                                    

with the balance being substantially iron, incidental impurities andaccessory elements.
 8. Steel according to claim 7, comprising thefollowing chemical composition in weight-%:

    ______________________________________                                                   0.75-0.85                                                                            C                                                                      3-5    Cr                                                                     2-4    Mo                                                                     2-6    W                                                                      0.7-1.5                                                                              V                                                                      2.5-10 Co                                                                     0.7-1.5                                                                              Nb                                                          ______________________________________                                    

with the balance being substantially iron, incidental impurities andaccessory elements.
 9. Steel according to claim 1, wherein tungsten iswholly or partly replaced by half the amount of molybdenum, ormolybdenum is wholly or party replaced by double the amount of tungsten.10. Steel according to claim 1, comprising the following chemicalcomposition in weight-%: 0.75% C, 0.2-0.5% Si, 0.2-0.5% Mn, 4% Cr, 3%Mo, 3% W, 1% V, 1% Nb, with the balance being substantially iron,incidental impurities and accessory elements.
 11. Steel according toclaim 1, comprising the following chemical composition in weight-%:0.73% C, 0.2-0.5% Si, 0.2-0.5% Mn, 4% Cr, 3% Mo, 3% W, 1% V, 1% Nb, withthe balance being substantially iron, incidental impurities andaccessory elements.
 12. Steel according to claim 1, comprising thefollowing chemical composition in weight-%: 0.70% C, 0.2-0.5% Si,0.2-0.5% Mn, 4% Cr, 3% Mo, 3% W, 1% V, 1% Nb, with the balance beingsubstantially iron, incidental impurities and accessory elements. 13.Steel according to claim 7, comprising the following chemicalcomposition in weight-%: 0.80% C, 0 2-0 5% Si, 0.2-0.5% Mn, 4% Cr, 3% MO3% W, 1% v, 1% Nb, 8% Co, with the balance being substantially iron,incidental impurities and accessory elements.
 14. An object manufacturedof a steel according to claim 1, wherein said steel of the object has amicrostructure containing 1-3 volume-% of secondarily precipitated M₂ C-and MC-carbides in a fine grain, substantially martensitic matrix which,besides the said M₂ C- and MC-carbides and niobium carbides, issubstantially free from carbides.
 15. The manufactured object accordingto claim 14, wherein said matrix has a microstructure in which theaustenite grains have a size corresponding to an Intercept>20 accordingto Snyder-Graff.
 16. The manufactured object according to claim 14,wherein the steel of said object has been hardened through solution heattreatment at a temperature between 925° and 1250° C., cooled to roomtemperature, and tempered at between 500° and 600° C.